N-channel MOSFETs comprising dual stressors, and methods for forming the same

ABSTRACT

The present invention relates to a semiconductor device including at least one n-channel field effect transistor (n-FET). Specifically, the n-FET includes first and second patterned stressor layers that both contain a carbon-substituted and tensilely stressed single crystal semiconductor. The first patterned stressor layer has a first carbon concentration and is located in source and drain (S/D) extension regions of the n-FET at a first depth. The second patterned stressor layer has a second, higher carbon concentration and is located in S/D regions of the n-FET at a second, deeper depth. Such an n-FET with the first and second patterned stressor layers of different carbon concentration and different depths provide improved stress profile for enhancing electron mobility in the channel region of the n-FET.

FIELD OF THE INVENTION

The present invention relates to complementary metal-oxide-semiconductor(CMOS) devices with enhanced carrier mobility. More specifically, thepresent invention relates to n-channel metal-oxide-semiconductor fieldeffect transistors (n-MOSFETs) having dual stressors for enhancingelectronic mobility in channel regions of such n-MOSFETs.

BACKGROUND OF THE INVENTION

Mechanical stresses within a semiconductor device substrate can be usedto modulate device performance. For example, in silicon, hole mobilityis enhanced when the silicon film is under compressive stress, while theelectron mobility is enhanced when the silicon film is under appropriatetensile stress. Therefore, compressive or tensile stress can beadvantageously created in the channel region of a p-MOSFET or ann-MOSFET in order to enhance the performance of such a device.

One conventional approach for creating a desirable stressed siliconchannel region is to form such a channel region directly on top of astress-inducing buffer layer. For example, a tensilely stressed siliconchannel layer can be formed by epitaxially growing silicon directly overa thick, relaxed SiGe buffer layer. The lattice constant of germanium isabout 4.2% greater than that of silicon, and the lattice constant of asilicon-germanium alloy is linear with respect to its germaniumconcentration. As a result, the lattice constant of a SiGe alloy withtwenty atomic percent of germanium is about 0.8% greater than thelattice constant of silicon. Epitaxial growth of silicon directly on topof such a SiGe buffer layer will yield a silicon channel layer undertensile stress, with the underlying SiGe buffer layer being essentiallyunstrained, or “relaxed”.

The use of such a strain-inducing SiGe layer has several inherentdisadvantages: (1) formation of relaxed SiGe buffer layer relies ondefect formation, and consequentially, the SiGe material has a highdefect density, which propagates into the silicon channel layerthereabove and poses significant challenges for device applications,such as control of leakage current and device yield, and (2) thepresence of the SiGe layer directly underneath the channel regioncreates processing issues, such as deleterious diffusion of germaniuminto the strained silicon channel, high resistance silicide formationand altered dopant diffusion.

There is a continuing need for improved semiconductor devices containinghigh performance MOSFET components.

SUMMARY OF THE INVENTION

The present invention provides an n-channel field effect transistor(n-FET) comprising dual stressors for applying desired tensile stress tothe channel region of the n-FET. The dual stressors in the n-FET of thepresent invention provide an improved stress profile in the channelregion of the n-FET, without otherwise compromising the deviceperformance of the n-FET.

In one aspect, the present invention relates to a semiconductor devicecomprising at least one n-channel field effect transistor (n-FET). Theat least one n-FET specifically comprises first and second patternedstressor layers, both of which comprise a carbon-substituted andtensilely stressed single crystal semiconductor. The first patternedstressor layer has a first substitutional carbon concentration and islocated in source and drain (S/D) extension regions of the n-FET. Thesecond patterned stressor layer has a second, higher substitutionalcarbon concentration and is located in S/D regions of the n-FET.

Preferably, but not necessarily, the first substitutional carbonconcentration ranges from about 0.2 atomic % to about 2.5 atomic %, andthe second, higher substitutional carbon concentration ranges from about0.5 atomic % to about 4 atomic %. More preferably, the firstsubstitutional carbon concentration ranges from about 0.5 atomic % toabout 2 atomic %, and the second, higher substitutional carbonconcentration ranges from about 0.8 atomic % to about 3 atomic %.

The first and second patterned stressor layers may be respectivelylocated in the S/D extension regions and the S/D regions atsubstantially the same depth or at significantly different depths. Inone specific embodiment of the present invention, the first patternedstressor layer is located in the S/D extension regions at a first,relatively shallow depth, and the second patterned stressor layer islocated in the S/D regions at a second, relatively deep depth. In analterative embodiment of the present invention, the first patternedstressor layer is located in the S/D extension regions at a first,relatively deep depth, and the second patterned stressor layer islocated in the S/D regions at a second, relatively shallow depth. In afurther alterative embodiment of the present invention, the first andsecond patterned stressor layers are respectively located in the S/Dextension regions and the S/D regions at substantially the same depths.

The first patterned stressor layer is preferably located in the S/Dextension regions of the n-FET at a first depth ranging from about 5 nmto about 80 nm, and more preferably from about 10 nm to about 50 nm. Thesecond patterned stressor layer is preferably located in the S/D regionsof the n-FET at a second depth ranging from about 10 nm to about 150 nm,and more preferably from about 20 nm to about 80 nm.

The first and second patterned stressor layers may comprise any suitablecarbon-substituted and tensilely stressed single crystal semiconductor.Preferably, the first and second patterned stressor layers both comprisetensilely stressed single crystal silicon with substitutional carbonatoms located therein.

In another aspect, the present invention relates to a method for forminga semiconductor device comprising:

forming a patterned gate stack over a substrate that comprises a singlecrystal semiconductor;

forming a first set of amorphous regions in the substrate using thepatterned gate stack as a mask, wherein the first set of amorphousregions comprise n-type source and drain (S/D) extension implants, andwherein the first set of amorphous regions further comprise implantedcarbon ions at a first carbon concentration;forming one or more masking structures along sidewalls of the patternedgate stack; forming a second set of amorphous regions in the substrateusing the patterned gate stack and the one or more masking structures asmasks, wherein the second set of amorphous regions comprise n-type S/Dimplants, and wherein the second set of amorphous regions furthercomprise implanted carbon ions at a second, higher carbon concentration;andannealing the substrate to recrystallize the first and second sets ofamorphous regions, thereby forming an n-channel field effect transistor(n-FET) comprising S/D extension regions and S/D regions, wherein theS/D extension regions comprise a first patterned stressor layer thatcomprises a carbon-substituted and tensilely stressed single crystalsemiconductor material and has a first substitutional carbonconcentration, and wherein the S/D regions comprise a second patternedstressor layer that also comprises the carbon-substituted and tensilelystressed single crystal semiconductor material but has a second, highersubstitutional carbon concentration.

In a further aspect, the present invention relates to a method forforming a semiconductor device comprising:

forming a patterned gate stack over a substrate that comprises a singlecrystal semiconductor; conducting a first masked pre-amorphizationimplantation to form a first set of amorphous regions in the substrate;

conducting n-type source and drain (S/D) extension implantation at thefirst set of amorphous regions;

conducting a first carbon implantation to implant a first dosage ofcarbon ions into the first set of amorphous regions;

forming one or more spacers along sidewalls of the patterned gate stack;

conducting a second masked pre-amorphization implantation to form asecond set of amorphous regions in the substrate;

conducting n-type S/D implantation at the second set of amorphousregions; conducting a second carbon implantation to implant a second,larger dosage of carbon ions into the second set of amorphous regions;and

annealing the substrate to recrystallize the first and second sets ofamorphous regions, thereby forming an n-channel field effect transistor(n-FET) comprising source/drain (S/D) extension regions and S/D regions,wherein the S/D extension regions comprises a first patterned stressorlayer that comprises a carbon-substituted and tensilely stressed singlecrystal semiconductor material and has a first substitutional carbonconcentration, and wherein the S/D regions comprises a second patternedstressor layer that also comprises the carbon-substituted and tensilelystressed single crystal semiconductor material but has a second, highersubstitutional carbon concentration.

The first and second carbon implantation steps may be carried out atapproximately the same energy level or at significantly different energylevels. In one specific embodiment of the present invention, the firstcarbon implantation step is carried out at a first, relatively lowenergy level, and the second carbon implantation step is carried out ata second, relatively high energy level. In an alternative embodiment ofthe present invention, the first carbon implantation step is carried outat a first, relatively high energy level, and the second carbonimplantation step is carried out at a second, relatively low energylevel. In a further alternative embodiment of the present invention, thefirst and second carbon implantation steps are carried out atapproximately the same energy level.

Preferably, but not necessarily, the first carbon implantation step isconducted at a first energy level ranging from about 0.5 KeV to about 15KeV, and the second carbon implantation step is conducted at a secondenergy level ranging from about 1 KeV to about 25 KeV. More preferably,the first energy level ranges from about 1 KeV to about 10 KeV, and thesecond energy level ranges from about 1 KeV to about 15 KeV.

The first dosage of carbon ions may range from about 1×10¹⁴/cm² to about1×10¹⁶/cm², and the second, higher dosage of carbon ions may range fromabout 5×10¹⁴/cm² to about 2×10¹⁶/cm². More preferably, the first dosagemay range from about 5×10¹⁴/cm² to about 5×10¹⁵/cm², and the second,higher dosage may range from about 1×10¹⁵/cm² to about 1×10¹⁶/cm².

Other aspects, features and advantages of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of an exemplary n-FET that comprisesdual Si:C stressors, according to one embodiment of the presentinvention.

FIGS. 2-4 show cross sectional views that illustrate the exemplaryprocessing steps for forming the n-FET of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide a thoroughunderstanding of the present invention. However, it will be appreciatedby one of ordinary skill in the art that the invention may be practicedwithout these specific details. In other instances, well-knownstructures or processing steps have not been described in detail inorder to avoid obscuring the invention.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” another element, there are no intervening elements present. It willalso be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

The term “Si:C” or “carbon-substituted single crystal silicon” as usedherein refers to single crystal silicon having substitutional carbonatoms located therein. The carbon-substituted single crystal silicon mayor may not contain interstitial carbon atoms (the difference betweensubstitutional carbon atoms and interstitial carbon atoms will beexplained in greater detail hereinafter). The substitutional carbonatoms and the silicon atoms in Si:C form a silicon-carbon alloy, whichis still a semiconductor material. The Si:C as used in the presentinvention is therefore distinguished from silicon carbide, which is adielectric material that contains a carbon-silicon compound. Similarly,the term “Ge:C” or “carbon-substituted single crystal germanium” as usedherein refers to a germanium-carbon alloy that contains single crystalgermanium having substitutional carbon atoms therein, instead of thecompound form of germanium carbide.

The improved FET device of the present invention as well as theexemplary processing steps for fabricating the same will now bedescribed in greater detail by referring to the accompanying FIGS. 1-4.

Reference is first made to FIG. 1, which shows an n-FET device having asource region 22, a drain region 24, a source extension region 26, adrain extension region 28 and a channel region 30 located in asemiconductor substrate 10. Dotted line 21 indicates the doping level inthe source and drain (S/D) extension regions 26 and 28, and line 23indicates the doping level in the S/D regions 22 and 24. A gatedielectric layer 32 is located directly over the channel region 30 ofthe n-FET, and a gate conductor 34 is located directly over the gatedielectric layer 32. The n-FET may further comprise silicide contactlayers 22A and 24A over the S/D regions 22 and 24 and a gate silicidecontact layer 34A over the gate conductor 34. Optional spacers 35 and 36can be, but are not necessarily, provided along sidewalls of the gateconductor 34 and above the S/D extension regions 26 and 28 of the n-FET.

The semiconductor substrate 10 may comprise any semiconductor materialincluding, but not limited to: Si, SiC, SiGe, SiGeC, Ge, GaAs, InAs,InP, as well as other III-V or II-VI compound semiconductors.Semiconductor substrate 10 may also comprise an organic semiconductor ora layered semiconductor such as Si/SiGe, a silicon-on-insulator (SOI) ora SiGe-on-insulator (SGOI). In some embodiments of the presentinvention, it is preferred that the semiconductor substrate 10 becomposed of a Si-containing semiconductor material, i.e., asemiconductor material that includes silicon. The semiconductorsubstrate 10 may be doped, undoped or contain doped and undoped regionstherein. The semiconductor substrate 10 may include one or more dopeddevice regions (not shown), which may have either the same or differentconductivities and/or doping concentrations. The doped device regionsare typically known as “wells”.

The semiconductor substrate 10 may be strained, unstrained, or containregions of strained and unstrained semiconductor materials therein.Moreover, the semiconductor substrate 10 may have a singlecrystallographic surface orientation or multiple crystallographicsurface orientations.

One or more isolation regions 12 are typically formed into thesemiconductor substrate 10 to provide isolation between adjacentdevices. The isolation regions 12 may be trench isolation regions orfield oxide isolation regions. The trench isolation regions can beformed utilizing a conventional trench isolation process well known tothose skilled in the art. For example, lithography, etching and fillingof the trenches with a trench dielectric may be used in forming thetrench isolation regions. Optionally, a liner may be formed in thetrenches prior to trench fill, a densification step may be performedafter the trench fill and a planarization process may follow the trenchfill as well. The field oxide isolation regions may be formed utilizinga so-called local oxidation of silicon process.

A first patterned stressor layer 14 comprising at least two isolatedportions (as shown in FIG. 1) is located in the S/D extension regions 26and 28 of the n-FET, and a second patterned stressor layer 16 alsocomprising at least two isolated portions (as shown in FIG. 1) islocated in the S/D regions 22 and 24 of the n-FET. Both the first andsecond patterned stressor layers 14 and 16 contain intrinsic tensilestress, so that the channel region 30 of the n-FET are “pulled” by thesetwo stressor layers 14 and 16 at both ends. Desired tensile stress istherefore generated in the channel region 30 of the n-FET for enhancingelectron mobility therein.

Both the first and second patterned stressor layers 14 and 16 comprise acarbon-substituted single crystal semiconductor material. Thecarbon-substituted single crystal semiconductor material has a latticeconstant smaller than that of the surrounding substrate material 10,thereby generating tensile stress therein due to the lattice mismatchbetween such a carbon-substituted single crystal semiconductor materialand the surrounding substrate material 10.

Suitable single crystal materials that can be carbon doped to form thecarbon-substituted and tensilely stressed single crystal materials ofthe present invention include, but are not limited to: silicon,germanium, GaAs, InP, and etc. Therefore, the first and second patternedstressor layers 14 and 16 may comprise any suitable material selectedfrom the group consisting of: Si:C, Ge:C, GaAs:C, InP:C, etc. In aparticularly preferred embodiment of the present invention, thesubstrate material 10 comprises Si, and both the first and secondpatterned stressor layers 14 and 16 comprise Si:C, which has a latticeconstant smaller than Si and is therefore tensilely stressed due to thelattice mismatch between Si:C and Si.

Because the first patterned stressor layer 14 is located in the S/Dextension regions 26 and 28 in close proximity to the channel region 30of the n-FET, significantly more electron mobility enhancement can beachieved in the channel region 30 of the n-FET of the present invention,in comparison with conventional n-FETs that only contain stressorstructures in their S/D regions.

The first and second patterned stressor layers 14 and 16 can be readilyformed by a solid phase epitaxy (SPE) process, which is effectuated byamorphization implantation, carbon implantation, and annealing. However,stacking faults (i.e., crystal defects) may be generated in the firstand second patterned stressor layers 14 and 16 during the SPE process,which can adversely affect the device performance of the n-FET.

The inventors of the present invention have discovered that the densityof the stacking faults generated during the SPE process correlates withthe substitutional carbon concentrations in the stressor layers 14 and16, i.e., the higher the substitutional carbon concentration, the higherthe density of stacking faults. Further, the inventors of the presentinvention have discovered that a large portion of the stacking faults iswithin the silicidation range, i.e., a large portion of the stackingfaults can be consumed by formation of surface silicide contact layers.

Therefore, the present invention solves the stacking defect problem byproviding a dual stressor configuration, in which the first patternedstressor layer 14 having a relatively low substitutional carbonconcentration is located in the un-silicidated S/D extension regions 26and 28, and the second patterned stressor layer 16 having a relativelyhigh substitutional carbon concentration is located in the silicided S/Dregions 22 and 24. Consequently, fewer or no stacking faults aregenerated in the first patterned stressor layer 14, due to therelatively low substitutional carbon concentration in layer 14. Althoughmore stacking faults are generated in the second patterned stressorlayer 16 due to the relatively high substitutional carbon concentrationin layer 16, most of the stacking faults so generated are subsequentlyconsumed by S/D silicidation when the S/D silicide contact layers 22Aand 24A are formed. Therefore, an enhanced stress profile can beachieved in the n-FET by using such a dual stressor configuration, whilelittle or no overall stacking faults are generated therein.

The relatively low substitutional carbon concentration in the firstpatterned stressor layer 14 preferably ranges from about 0.2 atomic % toabout 2.5 atomic %, and more preferably from about 0.5 atomic % to about2 atomic %. The relatively high substitutional carbon concentration inthe second patterned stressor layer 16 preferably ranges from about 0.5atomic % to about 4 atomic %, and more preferably from about 0.8 atomic% to about 3 atomic %.

Further, the inventors of the present invention have discovered that incarbon-substituted single crystal semiconductor materials, the amount ofsubstitutional carbon atoms (e.g., the carbon atoms that substitute thesilicon or germanium atoms and form a part of the crystal lattice insingle crystal silicon or germanium, in distinction with interstitialcarbon atoms that are present interstitially between crystal latticesbut do not constitute a part of the crystal lattice) increases with thetotal carbon concentration, but not linearly. In other words, morecarbon atoms are present in the interstitial sites between crystal unitcells when the total carbon concentration increases. The presence ofinterstitial carbon atoms in the S/D junctions, which are spaced apartfrom the channel region of the n-FET, has little or no adverse impact onthe n-FET device performance. However, the presence of interstitialcarbon atoms in the S/D extension junctions, which are locatedimmediately adjacent to the channel region of the n-FET, would causejunction leakage and carrier mobility degradation.

Such a problem associated with interstitial carbon can also be solved byincorporation of the dual stressor structure, as described hereinabove.Specifically, the first patterned stressor layer 14 having a relativelylow total carbon concentration is provided in the S/D extension regions26 and 28, and the second patterned stressor layer 16 having arelatively high total carbon concentration is provided in the S/Dregions 22 and 24. Consequently, the carbon atoms in the S/D extensionregions 26 and 28 are primarily substitutional carbon atoms, due to therelatively low total carbon concentration in the first patternedstressor layer 14. Since the first stressor layer 14 is located inproximity to the channel 30, it can effectively stress the channel evenwith a relatively low carbon concentration. In contrast, the carbonatoms in the S/D regions 22 and 24 may include a certain amount ofinterstitial carbon atoms, due to the relatively high total carbonconcentration in the second patterned stressor layer 16, but suchinterstitial carbon atoms are located in the S/D regions 22 and 24,which are far away from the channel region 30, and therefore have littleor no adverse impact on the n-FET device performance.

The first and second patterned stressor layer 14 may be located in anysuitable depth in the S/D extension and S/D regions. In one specificembodiment of the present invention, the first stressor layer 14 islocated in the S/D extension regions at a relatively shallow depth, andthe second stressor layer 16 is located in the S/D regions at arelatively deep depth, as shown in FIG. 1. Alternatively, the firststressor layer 14 is located in the S/D extension regions at arelatively deep depth, and the second stressor layer 16 is located inthe S/D regions at a relatively shallow depth. Further, the first andsecond stressor layers 14 and 16 may be located in the S/D extension andS/D regions at approximately the same depth. Preferably, the firstpatterned stressor layer 14 is located in the S/D extension regions 26and 28 at a depth ranging from about 5 nm to about 80 nm, and morepreferably from about 10 nm to about 50 nm, while the second patternedstressor layer 16 is located in the S/D regions 22 and 24 at a depthranging from about 10 nm to about 150 nm, and more preferably from about20 nm to about 80 nm.

The dual stressor configuration provided by the present inventiontherefore establishes an improved stress profile in the n-FET device,without otherwise compromising the device performance of the n-FET.

The n-FET device structures of the present invention can be readilyfabricated by any suitable method. Specifically, FIGS. 2-4 illustrate asequence of exemplary processing steps that can be used for fabricatingsuch an n-FET device structure, according to a specific embodiment ofthe present invention.

First, a semiconductor substrate 10, which may comprise any suitablesemiconductor material such as Si, SiC, SiGe, SiGeC, Ge, GaAs, InAs,InP, as well as other III-V or II-VI compound semiconductors, isprovided. Preferably, the semiconductor substrate 10 comprises asemiconductor material in its single crystal form.

Trench isolation regions 12 are then formed in the semiconductorsubstrate 10 to define a device region for an n-FET to be formed. Next,a patterned gate stack comprising a gate dielectric layer 32, a gateconductor 34 and an optional spacer 35 is formed over the semiconductorsubstrate 10. Formation of the trench isolation regions 12 and thepatterned gate stack can be readily carried out using conventional CMOSprocessing steps, which are not described in detail herein.

The patterned gate stack is then used as a mask for conducting,sequentially, a first masked pre-amorphization implantation (PAI), ann-type S/D extension implantation, an optional p-type halo implantation,and a first carbon implantation, thereby forming a first set ofamorphous regions 13 in the semiconductor substrate 10, as shown in FIG.2. Such amorphous regions 13 comprise n-type S/D extension implants(indicated by the S/D extension doping level 21), as well as implantedcarbon ions.

The n-type S/D extension implantation and the optional p-type haloimplantation can be readily carried out using conventional CMOSprocessing steps and are therefore not described in detail herein.

The first pre-amorphization implantation (PAI) is carried out toamorphize the single crystal semiconductor materials contained in theunmasked regions of the semiconductor substrate 10 by ion bombardment,thereby forming the amorphous regions 13 in the semiconductor substrate10 adjacent to the patterned gate stack. Any suitable ions can be usedfor performing the first PAI step, and the thickness of the amorphousregions 13 is substantially determined by the ion bombardment energy,the atomic mass, and the implanted dosage of the ions used. For singlecrystal silicon, it is preferable to use ions such as Si, Ge, Xe, Si, P,As, etc. However, it is also possible for the unmasked regions of thesemiconductor substrate 10 to be bombarded by other ions. The ion dosageused for the first PAI step is preferably selected to completelyamorphize regions 13 in the semiconductor substrate 10, so suchamorphous regions 13 can be recrystallized by solid phase epitaxy (SPE)on the semiconductor substrate 10 to again form a single crystalsemiconductor material at relatively low annealing temperatures. Whengermanium is used during the first PAI step, a germanium ion dosageranging from about 5×10¹³/cm² to about 1×10¹⁵/cm² is preferablyemployed.

Note that implantation of the extension dopant and the carbon atoms atcertain energy levels and certain dosages may function toself-amorphizing the implanted regions. In such an event, the first PAIstep is then not necessary.

The first carbon implantation can be conducted at any suitable energylevel. Preferably, but not necessarily, the first carbon implantation isconducted at a relatively low energy level (i.e., in comparison with thesecond, subsequent carbon implantation step), which may range from about0.5 KeV to about 15 KeV and more preferably from about 1 KeV to about 10KeV. Because the implantation energy level determines the implantationdepth, the carbon implants are preferably, but not necessarily, presentin the amorphous regions 13 at a first, relatively shallow depth (i.e.,in comparison with carbon implants formed by the second carbonimplantation step). Alternatively, the first carbon implantation can beconducted either at a relatively high energy level or at substantiallythe same energy level (i.e., in comparison with the second, subsequentcarbon implantation step), so that the carbon implants correspondinglyare present in the amorphous regions 13 at either a relatively deepdepth or approximately the same depth (i.e., in comparison with carbonimplants formed by the second carbon implantation step).

The first carbon implantation is typically conducted at a relatively lowcarbon dosage (i.e., in comparison with the second, subsequent carbonimplantation step), which may preferably range from about 1×10¹⁴/cm² toabout 1×10¹⁶/cm², and more preferably from about 5×10¹⁴/cm² to about5×10¹⁵/cm². Because the implantation dosage determines the implantedcarbon concentration, the carbon implants are present in the amorphousregions 13 at a first, relatively low concentration (i.e., in comparisonwith the second, subsequent carbon implantation step).

After formation of the carbon-substituted amorphous regions 13, asidewall spacer 36 is optionally formed over the semiconductor substratealong sidewalls of the patterned gate stack, as shown in FIG. 3.Alternatively, a sacrificial masking structure (not shown) can be formedalong sidewalls of the patterned gate stack, either in place of or inaddition to the offset spacer 36. Such a sacrificial masking structureis subsequently removed from the resulting n-FET structure after thefabrication process.

The patterned gate stack and the sidewall spacer 36 (or alternativelythe sacrificial offset masking structure) are then used jointly as masksfor conducting a second PAI, n-type S/D implantation, and a secondcarbon implantation, thereby forming a second set of amorphous regions15 in the semiconductor substrate 10, as shown in FIG. 3. Such a secondset of amorphous regions 15 comprises n-type S/D implants (indicated bythe S/D doping level 23) as well as implanted carbon ions.

The n-type S/D implantation can be readily carried out usingconventional CMOS processing steps and are therefore not described indetail herein.

The second PAI is carried out using the patterned gate stack and thesidewall spacer 36 (or alternatively the sacrificial offset maskingstructure) as masks to amorphize different regions of the semiconductorsubstrate 10 and form a second set of amorphous regions 15 in thesemiconductor substrate 10 adjacent to the first set of amorphousregions 13, as shown in FIG. 3. The same or different ions can be usedfor performing the first and second PAI steps, but it is important tonote that the implanted carbon concentration in the second set ofamorphous regions 15 is higher than that in the first set of amorphousregions 13.

The ion dosage used for the second PAI step is preferably selected tocompletely amorphize regions 15 in the semiconductor substrate 10, sosuch amorphous regions 15 can be recrystallized by solid phase epitaxy(SPE) on the semiconductor substrate 10 to again form a single crystalsemiconductor material. When germanium is used during the second PAIstep, a germanium ion dosage ranging from about 5×10¹³/cm² to about2×10¹⁵/cm² is preferably employed.

Note that implantation of the S/D dopant and the carbon atoms at certainenergy levels and certain dosages may function to self-amorphizing theimplanted regions. In such an event, the second PAI step is then notnecessary.

The second carbon implantation can be conducted at any suitable energylevel. Preferably, but not necessarily, the second carbon implantationis conducted at a relatively high energy level (i.e., in comparison withthe first carbon implantation step), which may range from about 1 KeV toabout 25 KeV, and more preferably from about 1 KeV to about 15 KeV.Because the implantation energy level determines the implantation depth,the carbon implants are preferably, but not necessarily, present in theamorphous regions 15 at a second, relatively deep depth (i.e., incomparison with carbon implants formed by the first carbon implantationstep). Alternatively, the second carbon implantation can be conductedeither at a relatively low energy level or at substantially the sameenergy level (i.e., in comparison with the first carbon implantationstep), so that the carbon implants correspondingly are present in theamorphous regions 15 at either a relatively shallow depth orapproximately the same depth (i.e., in comparison with carbon implantsformed by the first carbon implantation step).

The second carbon implantation is typically conducted at a relativelyhigh carbon dosage (in comparison with that of the first carbonimplantation), which may preferably range from about 5×10¹⁴/cm² to about2×10¹⁶/cm², and more preferably from about 1×10¹⁵/cm² to about1×10¹⁶/cm². As mentioned hereinabove, the implantation dosage determinesthe implant concentration. Therefore, the carbon implants are present inthe amorphous regions 15 at a second, relatively high concentration incomparison with those in the amorphous regions 13.

After formation of the first and second sets of carbon-substitutedamorphous regions 13 and 15, the amorphous regions 13 and 15 are thenrecrystallized by annealing, which effectuates solid phase epitaxy (SPE)and again form a single crystal semiconductor material (which is,however, doped with carbon atoms now) in regions 13 and 15 over theun-amorphized regions of the substrate 10 thereunder. The annealing canbe readily carried out in any manner, including, but not limited to:furnace anneal, rapid thermal anneal (RTA), flash anneal, and laserannealing. In a particularly preferred embodiment of the presentinvention, a flash anneal or laser anneal step is carried out forrecrystallizing the carbon-substituted amorphous regions 13 and 15.

Correspondingly, the first and second patterned stressor layers 14 and16, which comprise a carbon-substituted single crystal semiconductormaterial, are formed in the semiconductor substrate 10, as shown in FIG.4. Specifically, the first patterned stressor layer 14 has a first,relatively low substitutional carbon concentration, and the secondpatterned stressor layer 16 has a second, relatively high substitutionalcarbon concentration.

The first and second patterned stressor layers 14 and 16 may be locatedin the semiconductor substrate 10 at substantially the same depth or atsignificantly different depths. In the specific embodiment illustratedby FIGS. 1-4, the first patterned stressor layer is located in thesemiconductor substrate 10 at a first, relatively shallow depth, and thesecond patterned stressor layer is located in the semiconductorsubstrate 10 at a second, relatively deep depth. However, it is readilyunderstood that the present invention broadly covers other alternativeembodiments where the first patterned stressor layer 14 is located inthe semiconductor substrate 10 at a relatively deep depth or atapproximately the same depth in comparison with the second patternedstressor layer 16.

Further, the S/D implants, the extension implants, and the patternedgate stack conjunctionally define an n-FET with S/D regions 22 and 24,S/D extension regions 26 and 28, a channel region 30, and a patternedgate stack located over the channel region 30, as shown in FIG. 4. Thefirst patterned stressor 14 comprises two portions that are respectivelylocated at the S/D extension regions 26 and 28 of the resulting n-FET.The second patterned stressor 16 also comprises two portions, which arerespectively located at the S/D regions 22 and 24 of the resultingn-FET, as shown in FIG. 4.

Further, a silicidation process can be carried out to form the S/Dsilicide contact layers 22A and 22B and the gate silicide contact layer34A shown in FIG. 1, and conventional back-end-of-line (BEOL) processingsteps can also be carried out to complete the n-FET device structure.The S/D silicide contact layers 22A and 22B cover the second patternedstressor layer 16, and they may also cover at least a portion of thefirst patterned stressor layer 14. The silicidation and BEOL processesare both well known in the art and are therefore not described in detailherein.

The n-FET device of the present invention uses a dual stressor structureto apply desired tensile stress to the channel region of the n-FETdevice, while minimizing potential deleterious impacts of crystaldefects and interstitial carbon atoms on the n-FET device performance.Therefore, an improved stress profile is achieved in the n-FET device ofthe present invention, without otherwise compromising the n-FET deviceperformance.

Note that in these drawings, which are not drawn to scale, like and/orcorresponding elements are referred to by like reference numerals. It isfurther noted that in the drawings, only one FET is shown on asemiconductor substrate. Although illustration is made to such anembodiment, the present invention is not limited to the formation of anyspecific number of FETs on the semiconductor substrate.

While FIGS. 1-4 illustratively demonstrates an exemplary n-FET structureand exemplary processing steps for fabricating same, according tospecific embodiments of the present invention, it is clear that a personordinarily skilled in the art can readily modify the device structureand the process steps illustrated herein, for adaptation to specificapplication requirements, consistent with the above descriptions. Forexample, although FIGS. 1-4 only show the dual stressors of the presentinvention without any other stress-inducing structures, it is understoodthat such dual stressors can be readily used in combination with otherknown stress-inducing structures, such as stress-inducing liners or caplayers, for further improving the electron mobility in the n-FETs of thepresent invention. It should therefore be recognized that the presentinvention is not limited to the specific embodiment illustratedhereinabove, but rather extends in utility to any other modification,variation, application, and embodiment, and accordingly all such othermodifications, variations, applications, and embodiments are to beregarded as being within the spirit and scope of the invention.

1. A semiconductor device comprising at least one n-channel field effecttransistor (n-FET), said at least one n-FET comprising first and secondpatterned stressor layers that both comprise a carbon-substituted andtensilely stressed single crystal semiconductor, wherein the firstpatterned stressor layer has a first substitutional carbon concentrationand is located in source and drain (S/D) extension regions of the n-FET,wherein the second patterned stressor layer has a second, highersubstitutional carbon concentration and is located in S/D regions of then-FET.
 2. The semiconductor device of claim 1, wherein the firstsubstitutional carbon concentration ranges from about 0.2 atomic % toabout 2.5 atomic %, and wherein the second, higher substitutional carbonconcentration ranges from about 0.5 atomic % to about 4 atomic %.
 3. Thesemiconductor device of claim 1, wherein the first substitutional carbonconcentration ranges from about 0.5 atomic % to about 2 atomic %, andwherein the second, higher substitutional carbon concentration rangesfrom about 0.8 atomic % to about 3 atomic %.
 4. The semiconductor deviceof claim 1, wherein the first patterned stressor layer is located in theS/D extension regions of the n-FET at a first depth ranging from about 5nm to about 80 nm, and wherein the second patterned stressor layer islocated in the S/D regions of the n-FET at a second depth ranging fromabout 10 nm to about 150 nm.
 5. The semiconductor device of claim 4,wherein the first depth ranges from about 10 nm to about 50 nm, andwherein the second depth ranges from about 20 nm to about 80 nm.
 6. Thesemiconductor device of claim 1, wherein the first and second patternedstressor layers comprise carbon-substituted and tensilely stressedsingle crystal silicon.
 7. The semiconductor device of claim 1, whereinthe source and drain regions of the at least one n-FET comprise sourceand drain silicide layers that are located over the second patternedstressor layer and optionally at least a portion of the first patternedstressor layer.